Formation breakdown failures were early on identified as technical risk in hydraulic fracturing treatments of wells in the Sichuan basin, China. This study focuses on reducing the breakdown pressure while improving near-wellbore fracture geometries by a variety of perforation geometries as well as cyclic pressure ramp-ups. A total of six large scale polyaxial experiments on isotropic cement samples were conducted, in which the principal stresses were set to represent a horizontal wellbore in a normally faulted stress regime. The results show that breakdown pressures can be reduced by 25% with in-plane perforations or cyclic pressure ramping. The results further indicate that open hole and conventional plug-and-perf completions lead to more near wellbore fracture tortuosities, while transverse fractures can be initiated from the in-plane perforation geometry.
Most operators active in unconventional resource plays base their hydraulic fracturing treatment design on their experiences gained in North America's normally faulted marine basins. In areas with more challenging tectonic settings and deeper targets, however, hydraulic fracturing treatments need to be designed for higher breakdown pressures, higher average pumping pressures, and the risks of limited fracture height growth as well as pre-mature screen-outs.
In China's Sichuan Basin, breakdown pressures exceeding the design pressure rating of available standard pumping equipment (15 kpi surface pressure limit) have caused the failure of numerous hydraulic fracturing stages, or required additional perforations or sand jetting before the formation would break down.
In addition to issues related to breakdown pressure, non-radioactive tracer logs and micro-seismic data indicated very limited fracture heights. It might be possible that at certain formation layers, the hydraulic fractures were propagating horizontally. This hypothesis is reinforced by the closure stresses of several injection tests, which yielded a theoretical minimum horizontal stress equivalent to the overburden stress . This suggests a strike-slip (sH > sv > sh) to reversely faulted stress regime (sH > sh > sv). When stimulating horizontal wells drilled on sh azimuth in such settings, the hydraulic fractures initiate along the sides of the wellbore where the hoop-stress s? governed by sH >> sv is minimum. As the fracture then propagates into the principal stress governing zone away from the wellbore, two factors inhibit its re-orientate into the vertical plane: first, the marginal stress anisotropy sv - sh accommodates either orientation and therefore does not act as the required restoring force, and second, the laminated fabric of these shale formations favours horizontal fractures parallel to its bedding plane.